1. Field of the Invention
The present invention pertains to cleaning solder paste from stencils, screens, and tools used in producing surface mount technology circuit boards.
2. Background of the Invention
Stencils, screens, and tools used in manufacturing surface mount technology (S.M.T.) circuit boards require frequent cleaning because the solder paste which is deposited on circuit boards during manufacturing accumulates on these items through repeated use. Such solder paste buildup obstructs the smaller apertures on the stencils and screens and reduces the precision with which the tools may be used. In addition, occasionally misprinted boards are often discarded for lack of an effective and inexpensive method for cleaning the solder paste from the boards. Presently, stencils, screens, and tools (hereinafter usually referred to collectively as "stencils") are typically cleaned by manually applying a halogenated hydrocarbon-based solvent and using a stiff brush to scrape the solder paste from the stencil surfaces. In the alternative, stencils are cleaned using machines having a conveyor assembly which carries the stencils past fixed spray nozzles.
The above methods for cleaning S.M.T. stencils require that solvent be used at elevated temperatures, usually on the order of approximately thirty-eight (38) degrees Celsius. This is necessary because of the inefficient cleaning effect of the solvent when used in such a manner. Use of solvent at such temperatures causes substantial evaporative loss of solvent, environmental injury resulting from solvent release into the atmosphere, and worker exposure to solvent vapors. Furthermore, exposure of the material by which S.M.T. stencils are usually suspended within their frames to heated solvents tend to damage that material thereby requiring more frequent replacement of the stencil/frame combinations than would be necessary if solvent were used at or below room temperature.
Solder paste contains lead which is removed from S.M.T. stencils during cleaning. Worker exposure to lead is a health concern which is addressed by strict government standards. Cleaning S.M.T. stencils by the above-referenced methods do little, if any, to address the lead exposure or collection problems.